The invention relates to a process and a device for detecting foreign matter in a fibre assembly which is moved lengthwise.
A process of this type and a device of this type are known from CH 674 379, in which a textile fibre material is illuminated with multicoloured or white light and an image of the yarn is produced on two sensors which are each only sensitive to one colour. The output signals of the sensors are guided onto an electronic differential circuit. Colour changes which are caused by foreign fibres in the yarn lead to a spontaneous change of the output signal from the differential circuit if the colour differs from the raw cotton.
A further process and a further device are known from EP 0 652 432. A textile fabric moved lengthwise is also illuminated here by a polychromatic light source and the reflected light is simultaneously detected on at least two wavelengths. Wavelengths in the near infrared range should also be detected in order to detect foreign matter, the colour of which differs from the basic colour of the yarn to be checked.
A disadvantage of these known processes and devices is that reliable differentiation of foreign matter such as, for example, polypropylene, is impossible in the spectral range of visible light. However, as polypropylene is very widely used as packaging material for cotton bales and the covers of the bales are not always removed with the necessary care, allowance has to be made for parts of the polypropylene covers being mixed with the cotton and appearing at any time in the processing of the cotton and being able to impair the product. Although processes are known from the plastics industry which are based on the high absorption of the Cxe2x80x94H bond of the plastics materials, the obvious attempt to detect polypropylene in cotton, in that polypropylene is shown in cotton by absorption of the radiation at 3.43 micrometers, is inconclusive, as polypropylene and cotton both appear as dark areas. Perfect detection of polypropylene is difficult under these circumstances and can lead to false conclusions on its presence.
The invention as characterised in the claims achieves the object of providing a process and device which avoid these disadvantages and allow improved, simplified and rapid detection of polypropylene in fibres which are combined as yarn, nonwoven fabric or flocks for textile products.
This is achieved in that the reflection with different strengths of fibres made of cotton, wool, etc. and of foreign matter such as, for example, polypropylene is deliberately exploited. We refer here to the reflection of beams on the actual basic materials of which the fibre assembly on the one hand and the foreign matter on the other hand consist. This is in contrast to known processes which carry out detection on the basis of properties of additives, such as, for example, dyes. Dyes absorb and reflect in different wavelength ranges to the basic materials such as, for example, cellulose or manmade fibres such as, for example, nylon as basic material for the fibres of the fibre assembly. The invention exploits the fact that the reflection of infrared radiation by the fibres and by the foreign matter differs more in certain wavelength ranges than in other wavelength ranges. However, depending on the batch, this can have more or less success. According to the present invention it is proposed to coordinate various points of view so the desired success is achieved. Firstly, radiation should be used with at least one wavelength at which the reflection on the basic materials of the fibres and on the basic materials of the foreign matter produces values which are as different as possible to allow a good selection. Secondly, at least one wavelength of the radiation used should advantageously be selected in such a way that the fibres, which should be the basic material, carrier or a type of background for the foreign matter, reflect the radiation as little as possible or not at all. This then allows the fibre assembly and therefore the basic material to be shown against a background which is such that it does not reflect radiation of this type. With this procedure a process is obtained in which the test piece is subjected to radiation of a specific wavelength and in the process only the foreign matter, here in particular the polypropylene, appears as a light patch. In the process there can be no distinction between the background and the test piece. This means that the radiation is adapted to the basic material thereof in such a way that it absorbs this radiation and that polypropylene at least partially reflects the radiation, the background appearing identical to the basic body of the fibre assembly to the sensor, i.e. dark. This can be achieved in that, for example, the background absorbs the radiation in an absorber, or reflects or diffuses it. An optical element, for example a dielectric filter, the reflection of which has been adapted, can also be used as a background.
In brief, the fibre assembly should be subjected to infrared radiation and the reflected radiation should be measured from a limited wavelength range, values which substantially differ from a basic value indicating a foreign matter. The limited wavelength range can be produced by filtering the radiation or directly by a suitable radiation source. The limited wavelength should be adapted to an absorption band of the basic material, for example to cellulose in the case of natural fibres. An absorption band around a wavelength of approximately 2.95 micrometers is particularly favourable, as cellulose absorbs in this range. In this range, cellulose appears xe2x80x9cblackxe2x80x9d and the background can easily be adapted as is necessary for the independence of the measurement from the diameter of the fibre assembly. Two wavelengths can also be selected which are adapted to the basic material of the fibres and the foreign matter in such a way that the foreign matter becomes noticeable in different ways in the two wavelengths.
The device according to the invention therefore has a suitable radiation source, a means for limiting the wavelength range and a detector. A means of this type is, for example, a filter which separates those portions of the radiation which have an undesired wavelength. An image-forming system can be, but need not be provided. A circuit for evaluating the signal emitted by the detector is connected thereto.
In a particular embodiment, the process according to the invention can be designed in such a way that the reflected radiation is divided into at least two beams and is filtered, the filtered portion being measured for each beam and the measured values related to one another or compared to indicate foreign matter.
The advantages achieved by the invention are in particular, that a signal which is to be unambiguously interpreted is produced thereby and reliably indicates whether there is any foreign matter of this type present or not. The invention also allows the process to be used in various wavelengths and two signals to be produced in this way which then indicate foreign matter if both suggest the same conclusion. Measurement in two wavelengths also allows foreign matter to be detected without adaptation of the background to the test piece. If adaptation is possible in the two wavelengths, this can be an advantage. Therefore, a large difference in the quantity of reflected radiation in the ranges of a single wavelength and a smaller or opposite difference in the ranges of other wavelengths can be compared and exploited in such a way that a definite statement is also possible. A further advantage is that the device can be constructed from components which are known per se and commercially available. As neither the dye nor an additive to the basic material is to be detected, there is no false indication of defects in the case of vegetable foreign matter or impurities if, in the case of vegetable matter and natural fibres, the basic material which is to react to the radiation is the same.
With this process and device foreign matter or impurities may be detected in different types of fibre assemblies such as slivers, webs, flakes and so on.
The invention will be described in more detail hereinafter with the aid of an embodiment and with reference to the accompanying drawings, in which:
FIG. 1 is a schematic view of a first embodiment of the device according to the invention,
FIG. 2 is a schematic view of a second embodiment of the device according to the invention,
FIG. 3 is a view of the reflection behaviour of fibres and foreign matter,
FIG. 4 is a schematic view of the characteristics of a filter used in the device and
FIG. 5 shows possible courses of output signals.
FIG. 6 shows a further embodiment of a device according to the invention.